The present invention is described in the following three papers published by the applicants.
The first paper is N. Praphairaksit et al., xe2x80x9cReduction of Space Charge Effects in Inductively Coupled Plasma Mass Spectrometry Using a Supplemental Electron Source inside the Skimmer: Ion Transmission and Mass Spectral Characteristicsxe2x80x9d, Analytical Chemistry, Vol. 72, No. 11, Jun. 1, 2000, pp. 2356-2361 (the Praphairaksit I paper). An earlier version of this paper is included in U.S. provisional application Serial No. 60/175,688 referred to above as Appendix I.
The second paper is N. Praphairaksit et al., xe2x80x9cAttenuation of Matrix Effects in Inductively Coupled Plasma Mass Spectrometry with a Supplemental Electron Source inside the Skimmerxe2x80x9d, Analytical Chemistry, Vol. 72, No. 11, Jun. 1, 2000, pp. 2351-2355 (the Praphairaksit II paper). An earlier version of this paper is included in U.S. provisional application Serial No. 60/175,688 referred to above as Appendix II.
The third paper is N. Praphairaksit et al., xe2x80x9cReduction of Mass Bias and Matrix Effects in Inductively Coupled Plasma Mass Spectrometry with a Supplemental Electron Source in a Negative Extraction Lensxe2x80x9d, Analytical Chemistry, Vol. 72, No. 18, Sep. 15, 2000, pp. 4435-4440 (the Praphairaksit III paper). An earlier version of this paper is included in U.S. provisional application Serial No. 60/175,688 referred to above as Appendix III.
The Praphairaksit I, Praphairaksit II, and Praphairaksit III papers referred to above are incorporated herein by reference in their entirety.
1. Field of the Invention
The present invention is directed to mass spectrometers, and in particular to mass spectrometers wherein space charge effects occur in a sample beam due to an excess of positive ions in the sample beam. The present invention is particularly useful in inductively coupled plasma-mass spectrometers (ICP-MS).
2. Description of the Related Art
FIG. 1 shows a prior-art mass spectrometer which is basically the same as that described in K. Hu et al., xe2x80x9cInductively Coupled Plasma Mass Spectrometry with an Enlarged Sampling Orifice and Offset Ion Lens. I. Ion Trajectories and Detector Performancexe2x80x9d, Journal of the American Society for Mass Spectrometry, Vol. 4, 1993, pp. 16-27 (the Hu I reference), and K. Hu et al., xe2x80x9cInductively Coupled Plasma Mass Spectrometry with an Enlarged Sampling Orifice and Offset Ion Lens. II. Polyatomic Ion Interferences and Matrix Effectsxe2x80x9d, Journal of the American Society for Mass Spectrometry, Vol. 4, 1993, pp. 28-37 (the Hu II reference). One of the authors of the Hu I and Hu II references, Robert S. Houk, is also one of the applicants of the present application. The Houk I and Houk II references are incorporated herein by reference in their entirety.
The prior-art mass spectrometer in FIG. 1 includes an ion source 1, a sampling interface 2, and a mass analyzer 3.
In the Hu I and Hu II references, ion source 1 is an inductively coupled plasma ion source, and mass analyzer 3 is a quadrupole mass analyzer.
Sampling interface 2 includes a sampler 4, a skimmer 5, an ion lens 6, and a differential pumping plate 7. Ion lens 6 includes seven electrodes 8, 9, 10, 11, 12, 13, and 14, which are typically DC electrodes.
In the Hu I and Hu II references, a positive voltage +V1 is applied to a first electrode 8 of ion lens 7, meaning a voltage that is more positive than a potential of skimmer 5.
First electrode 8 may be a cylindrical electrode having holes in it as indicated by the dashed lines in FIG. 1, and may be formed of a mesh.
Ion source 1 generates a quasineutral beam 15 of positive ions and electrons wherein the total positive charge of the positive ions is substantially equal to the total negative charge of the electrons. Sampling interface 2 extracts a portion of quasineutral beam 15 to form a sample beam 16 which is analyzed by mass analyzer 3.
Quasineutral beam 15 and sample beam 16 are shown schematically in FIG. 1 as a single line representing the center lines of these two beams. The shapes of quasineutral beam 15 and sample beam 16 and the trajectories of the positive ions and the electrons in these two beams are described in the Hu I and Hu II references, and elsewhere in the prior art, for example, in H. Niu et al., xe2x80x9cFundamental aspects of ion extraction in inductively coupled plasma mass spectrometryxe2x80x9d, Spectrochimica Acta Part B, Vol. 51, 1996, pp. 779-815 (the Niu reference). One of the authors of the Niu reference, Robert S. Houk, is also one of the applicants of the present application. The Niu reference is incorporated herein by reference in its entirety.
As sample beam 16 passes through sampler 4 and skimmer 5, it is initially a quasineutral beam of positive ions and electrons wherein the total positive charge of the positive ions is substantially equal to the total negative charge of the electrons. However, as sample beam 16 travels downstream from skimmer 5 towards ion lens 6, it changes to a positively charged beam with an excess of positive ions, causing space charge effects to develop in sample beam 16. The reasons for this are described in detail in the Praphairaksit I and Praphairaksit II papers and the Hu I, Hu II, and Niu references.
However, a simplified explanation of the reasons for this is that electrons diffuse away from sample beam 16 as it travels downstream from skimmer 5 towards ion lens 6, creating an excess of positive ions in sample beam 16 and causing sample beam to become positively charged. This causes a space charge field to develop in sample beam 16, thereby causing space charge effects to develop in sample beam 16 as it travels downstream from skimmer 5 towards ion lens 6. Other fundamental reasons for preferential loss of electrons, in place of or in addition to diffusion, are also possible.
The space charge effects which develop in sample beam 16 have numerous disadvantages, and adversely affect the performance of the prior-art mass spectrometer in FIG. 1 as described in detail in the Praphairaksit I, Praphairaksit II, and Praphairaksit III papers and the Niu reference.
FIG. 2 is a diagram showing some consequences of these space charge effects, wherein (xe2x88x92) denotes electrons, (+) denotes light analyte ions, such as Li, and (+) denotes heavy analyte ions, such as U. As shown in FIG. 2, the electrons (xe2x88x92) diffuse away from sample beam 16 as it travels downstream from skimmer 5 towards ion lens 6, creating an excess of positive ions in sample beam 16 and causing sample beam 16 to become positively charged.
For the reasons discussed in detail in the Praphairaksit I, Praphairaksit II, and Praphairaksit III papers and the Niu reference, this causes sample beam 16 to defocus as it travels downstream from skimmer 5 towards ion lens 6, decreasing the number of the light analyte ions (+) and the heavy analyte ions (+) which are available to enter first electrode 8 of ion lens 6. Also, this causes the light analyte ions (+) to defocus to a greater extent than the heavy analyte ions (+), such that the ratio of the abundance (or number density) of the light analyte ions (+) to the abundance of the heavy analyte ions (+) in the portion of sample beam 16 which actually enters first electrode 8 of ion lens 6 is less than the actual ratio of the abundance of the light analyte ions (+) to the abundance of the heavy analyte ions (+) in sample beam 16 where it enters skimmer 5. These effects are particularly troublesome when attempting to measure a small amount of a light element such as Li, in a matrix of a heavy element, such as U, especially when ion source 1 has a high temperature which induces a large variation of ion kinetic energy with ion mass and/or ion mass-to-charge ratio, as described in the Niu reference.
One method of reducing space charge effects in a mass spectrometer is disclosed in S. Tanner et al., xe2x80x9cReduction of Space Charge Effects Using a Three-Aperture Gas Dynamic Vacuum Interface for Inductively Coupled Plasma-Mass Spectrometryxe2x80x9d, Applied Spectroscopy, Vol. 48, No. 11, 1994, pp. 1367-1372. This method uses a three-aperture sampling interface including a sampler, a skimmer downstream from the sampler, and a third aperture downstream from the skimmer which is offset from a beam axis defined by apertures in the sampler and the skimmer.
Another method of reducing space charge effects in a mass spectrometer is disclosed in E. Denoyer et al., xe2x80x9cDetermination of Trace Elements in Uranium: Practical Benefits of a New ICP-MS Lens Systemxe2x80x9d, Atomic Spectroscopy, Vol. 16, No. 1, January/February 1995, pp. 1-6. This method uses a sampling interface including a sampler, a skimmer downstream from the sampler, a grounded shadow stop downstream from the skimmer, and a single cylindrical ion lens downstream from the shadow stop. A voltage applied to the ion lens is dynamically varied in accordance with the mass-to-charge ratio of the ion being analyzed.
The applicants of the present application have invented another method of reducing space charge effects in a mass spectrometer which involves adding electrons to a sample beam having an excess of positive ions to reduce space charge repulsion between the positive ions in the sample beam, thereby reducing space charge effects in the sample beam and producing a sample beam having reduced space charge effects.
The present invention is directed to a mass spectrometer including an ion source which generates a beam including positive ions, a sampling interface which extracts a portion of the beam from the ion source to form a sample beam that travels along a path and has an excess of positive ions over at least part of the path, thereby causing space charge effects to occur in the sample beam due to the excess of positive ions in the sample beam, an electron source which adds electrons to the sample beam to reduce space charge repulsion between the positive ions in the sample beam, thereby reducing the space charge effects in the sample beam and producing a sample beam having reduced space charge effects, and a mass analyzer which analyzes the sample beam having reduced space charge effects.